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                <p>我们都知道反射是框架设计的灵魂，是必须要掌握的内容，华强都懂了，你还不懂吗，今天我就来带你揭开反射的真面目，就问你看不看吧。</p>
<h2 id="为什么要使用反射"><a href="#为什么要使用反射" class="headerlink" title="为什么要使用反射"></a>为什么要使用反射</h2><p>我们先想一下为什么要有反射，看下面的例子：</p>
<p>假如我们有一个接口 X 及其方法 test，和两个对应的实现类 A、B：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token punctuation">&#123;</span>
    
    <span class="token keyword">interface</span> <span class="token class-name">X</span> <span class="token punctuation">&#123;</span>
     <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
 <span class="token punctuation">&#125;</span>

    <span class="token keyword">class</span> <span class="token class-name">A</span> <span class="token keyword">implements</span> <span class="token class-name">X</span><span class="token punctuation">&#123;</span>
        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
             <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"I am A"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">class</span> <span class="token class-name">B</span> <span class="token keyword">implements</span> <span class="token class-name">X</span><span class="token punctuation">&#123;</span>
        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"I am B"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>我们正常使用哪个实现类就直接 new 一个就好了，看下面的代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token punctuation">&#123;</span>    
 	<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">X</span> a <span class="token operator">=</span> <span class="token function">create1</span><span class="token punctuation">(</span><span class="token string">"A"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        a<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">X</span> b <span class="token operator">=</span> <span class="token function">create1</span><span class="token punctuation">(</span><span class="token string">"B"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        b<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">X</span> <span class="token function">create1</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>name<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">"A"</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">A</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span>name<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">"B"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
            <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">B</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
        <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>如果按照这种写法，如果有很多个不同的X的实现类，就要写很多个if语句来创建不同实现类的对象，我们看看反射是如何做的：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token punctuation">&#123;</span>
    
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
 		<span class="token class-name">X</span> a <span class="token operator">=</span> <span class="token function">create2</span><span class="token punctuation">(</span><span class="token string">"A"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        a<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">X</span> b <span class="token operator">=</span> <span class="token function">create2</span><span class="token punctuation">(</span><span class="token string">"B"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        b<span class="token punctuation">.</span><span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
    
 	<span class="token comment">// 使用反射机制</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">X</span> <span class="token function">create2</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> <span class="token keyword">class</span> <span class="token operator">=</span> <span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">X</span> x <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">X</span><span class="token punctuation">)</span> <span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span> x<span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>向 <code>create2()</code> 方法传入包名和类名，通过反射机制动态的加载指定的类，然后再实例化对象。</p>
<p>看完上面这个例子，相信你对反射有了一定的认识。反射拥有以下四大功能：</p>
<ul>
<li>在运行时（动态编译）获知任意一个对象所属的类。</li>
<li>在运行时构造任意一个类的对象。</li>
<li>在运行时获知任意一个类所具有的成员变量和方法。</li>
<li>在运行时调用任意一个对象的方法和属性。</li>
</ul>
<p>上述这种<strong>「动态获取信息、动态调用对象的方法」</strong>的功能称为 Java 语言的反射机制。</p>
<h2 id="Class-类是个什么东东"><a href="#Class-类是个什么东东" class="headerlink" title="Class 类是个什么东东"></a>Class 类是个什么东东</h2><p>要想理解反射，首先要理解 <code>Class</code> 类，因为 <code>Class</code> 类是反射实现的基础。</p>
<p>我们看看 JDK 中Class 类的源码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token keyword">implements</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span><span class="token punctuation">,</span>
                              <span class="token class-name">GenericDeclaration</span><span class="token punctuation">,</span>
                              <span class="token class-name">Type</span><span class="token punctuation">,</span>
                              <span class="token class-name">AnnotatedElement</span> <span class="token punctuation">&#123;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>在程序运行期间，JVM 始终为所有的对象维护一个被称为<strong>运行时的类型标识</strong>，这个信息跟踪着每个对象所属的类的完整结构信息，包括包名、类名、实现的接口、拥有的方法和字段等。可以通过专门的 Java 类访问这些信息，这个类就是 <code>Class</code> 类。我们可以把 <code>Class</code> 类理解为<strong>类的类型</strong>，一个 <code>Class</code> 对象，称为类的类型对象，<strong>一个 <code>Class</code> 对象对应一个加载到 JVM 中的一个 <code>.class</code> 文件</strong>。</p>
<p>在通常情况下，一定是先有类再有对象。以下面这段代码为例，类的正常加载过程是这样的：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>util<span class="token punctuation">.</span></span><span class="token class-name">Date</span><span class="token punctuation">;</span> <span class="token comment">// 先有类</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">Date</span> date <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 后有对象</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>date<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>首先 JVM 会将你的代码编译成一个 <code>.class</code> 字节码文件，然后被类加载器（Class Loader）加载进 JVM 的内存中，<strong>同时会创建一个 <code>Date</code> 类的 <code>Class</code> 对象存到堆中</strong>（注意这个不是 new 出来的对象，而是类的类型对象）。JVM 在创建 <code>Date</code> 对象前，会先检查其类是否加载，寻找类对应的 <code>Class</code> 对象，若加载好，则为其分配内存，然后再进行初始化 <code>new Date()</code>。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327102540081.png" alt="image-20220327102540081" style="zoom:80%;" />

<p>需要注意的是，<strong>每个类只有一个 <code>Class</code> 对象</strong>，也就是说如果我们有第二条 <code>new Date()</code> 语句，JVM 不会再生成一个 <code>Date</code> 的 <code>Class</code> 对象，因为已经存在一个了。这也使得我们可以利用 <code>==</code> 运算符实现两个类对象比较的操作：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>date<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token class-name">Date</span><span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>在加载完一个类后，内存中就产生了一个<code>Class</code> 对象，这个对象就包含了完整的类的结构信息，<strong>我们可以通过这个 <code>Class</code> 对象看到类的结构</strong>，就好比一面镜子。所以我们形象的称之为：反射。</p>
<blockquote>
<p>在通常情况下，是先有类再有对象，我们把这个通常情况称为 “正”。那么反射中的这个 “反” 我们就可以理解为根据对象找到对象所属的类（对象的出处）</p>
</blockquote>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Date</span> date <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>date<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// "class java.util.Date"</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>通过反射，也就是调用了 <code>getClass()</code> 方法后，我们就获得了 <code>Date</code> 类对应的 <code>Class</code> 对象，看到了 <code>Date</code> 类的结构，输出了 <code>Date</code> 对象所属的类的完整名称，即找到了对象的出处。当然，获取 <code>Class</code> 对象的方式不止这一种。</p>
<h2 id="获取-Class-类对象的方式"><a href="#获取-Class-类对象的方式" class="headerlink" title="获取 Class 类对象的方式"></a>获取 Class 类对象的方式</h2><p>从 <code>Class</code> 类的源码可以看出，它的构造函数是私有的，也就是说只有 JVM 可以创建 <code>Class</code> 类的对象，我们不能像普通类一样直接 new 一个 <code>Class</code> 对象。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">private</span> <span class="token class-name">Class</span><span class="token punctuation">(</span><span class="token class-name">ClassLoader</span> loader<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        classLoader <span class="token operator">=</span> loader<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>我们只能通过已有的类或对象来得到一个 <code>Class</code>类对象，Java 提供了四种方式：</p>
<p><strong>第一种</strong>：</p>
<pre class="line-numbers language-none"><code class="language-none">Class clazz &#x3D; 类名.class;<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<blockquote>
<p>但是我们一般是不知道具体类的，基本都是通过遍历包下面的类来获取 Class 对象，通过此方式获取 Class 对象不会进行初始化。</p>
</blockquote>
<p><strong>第二种</strong>：</p>
<pre class="line-numbers language-none"><code class="language-none">Class clazz &#x3D; Class.forName(&quot;com.xxx.类名&quot;);<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>这个 forName 方法底层是调用的 forName0，源码如下：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> <span class="token function">forName</span><span class="token punctuation">(</span><span class="token class-name">String</span> className<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">ClassNotFoundException</span> <span class="token punctuation">&#123;</span> 
    <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> caller <span class="token operator">=</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>      
    <span class="token keyword">return</span> <span class="token function">forName0</span><span class="token punctuation">(</span>className<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">,</span> <span class="token class-name">ClassLoader</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span>caller<span class="token punctuation">)</span><span class="token punctuation">,</span> caller<span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>第 2 个 <code>boolean</code> 参数表示类是否需要初始化，默认是需要初始化。一旦初始化，就会触发目标对象的 <code>static</code> 块代码执行，<code>static</code> 参数也会被再次初始化。</p>
<p><strong>第三种</strong>：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Date</span> date <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token class-name">Class</span> clazz <span class="token operator">=</span> date<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 获取该对象实例的 Class 类对象</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<blockquote>
<p>通过对象实例 <code>instance.getClass()</code> 获取。</p>
</blockquote>
<p><strong>第四种</strong>：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Class</span> clazz <span class="token operator">=</span> <span class="token class-name">ClassLoader<span class="token punctuation">.</span>LoadClass</span><span class="token punctuation">(</span><span class="token string">"com.xxx.类名"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<blockquote>
<p>通过类加载器获取 Class 对象不会进行初始化，意味着不进行包括初始化等一些列步骤，静态块和静态对象不会得到执行。</p>
</blockquote>
<h2 id="反射构造一个类的实例"><a href="#反射构造一个类的实例" class="headerlink" title="反射构造一个类的实例"></a>反射构造一个类的实例</h2><p>上面我们介绍了获取 <code>Class</code> 类对象的方式，那么成功获取之后，我们就需要构造对应类的实例。下面介绍三种方法，第一种最为常见，最后一种大家稍作了解即可。</p>
<p><strong>使用 Class.newInstance</strong></p>
<p>举个具体的例子方便大家理解：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Date</span> date1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Date</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Class</span> clazz <span class="token operator">=</span> date1<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Date</span> date2 <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 创建一个与 clazz 具有相同类类型的实例</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span></span></code></pre>

<p>需要注意的是，<code>newInstance</code>方法调用默认的构造函数（无参构造函数）初始化新创建的对象。如果这个类没有默认的构造函数， 就会抛出一个异常。源码分析如下：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InstantiationException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalAccessException</span> <span class="token punctuation">&#123;</span>  
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">getSecurityManager</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>   
        <span class="token function">checkMemberAccess</span><span class="token punctuation">(</span><span class="token class-name">Member</span><span class="token punctuation">.</span>PUBLIC<span class="token punctuation">,</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
    <span class="token punctuation">&#125;</span>   
    <span class="token keyword">if</span> <span class="token punctuation">(</span>cachedConstructor <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>   
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">this</span> <span class="token operator">==</span> <span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>     
            <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalAccessException</span><span class="token punctuation">(</span><span class="token string">"Can not call newInstance() on the Class for java.lang.Class"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p> <strong>通过反射先获取构造方法再调用</strong></p>
<p>由于不是所有的类都有无参构造函数又或者类构造器是 <code>private</code> 的，在这样的情况下，如果我们还想通过反射来实例化对象，<code>Class.newInstance</code> 是无法满足的。此时，我们可以使用 <code>Constructor</code> 的 <code>newInstance</code> 方法来实现，先获取构造函数，再执行构造函数。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token class-name">Object</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> initargs<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">InstantiationException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalAccessException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">,</span> <span class="token class-name">InvocationTargetException</span> <span class="token punctuation">&#123;</span> 
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>override<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>       
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">quickCheckMemberAccess</span><span class="token punctuation">(</span>clazz<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>       
            <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> caller <span class="token operator">=</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>       
            <span class="token function">checkAccess</span><span class="token punctuation">(</span>caller<span class="token punctuation">,</span> clazz<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">;</span>    
        <span class="token punctuation">&#125;</span>  
    <span class="token punctuation">&#125;</span>   
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>clazz<span class="token punctuation">.</span><span class="token function">getModifiers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> <span class="token class-name">Modifier</span><span class="token punctuation">.</span>ENUM<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token string">"Cannot reflectively create enum objects"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token class-name">ConstructorAccessor</span> ca <span class="token operator">=</span> constructorAccessor<span class="token punctuation">;</span>   
    <span class="token comment">// read volatile   </span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>ca <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>  
        ca <span class="token operator">=</span> <span class="token function">acquireConstructorAccessor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">&#125;</span>   
    <span class="token annotation punctuation">@SuppressWarnings</span><span class="token punctuation">(</span><span class="token string">"unchecked"</span><span class="token punctuation">)</span>  
    <span class="token class-name">T</span> inst <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">)</span> ca<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span>initargs<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token keyword">return</span> inst<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>从上面代码很容易看出，<code>Constructor.newInstance</code> 是可以携带参数的，而 <code>Class.newInstance</code> 是无参的，这也就是为什么它只能调用无参构造函数的原因了。</p>
<blockquote>
<p>大家不要把这两个 <code>newInstance</code> 方法弄混了。如果被调用的类的构造函数为默认的构造函数，采用<code>Class.newInstance()</code> 是比较好的选择， 一句代码就 OK；如果需要调用类的带参构造函数、私有构造函数等， 就需要采用 <code>Constractor.newInstance()</code>。</p>
</blockquote>
<p><code>Constructor.newInstance</code> 是执行构造函数的方法。我们来看看获取构造函数可以通过哪些渠道，作用如其名，以下几个方法都比较好记也容易理解，返回值都通过 <code>Cnostructor</code> 类型来接收。</p>
<p>那怎么获取到构造函数了，主要有下面几种方法：</p>
<p>1）获取所有”公有的”构造方法</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Constructor</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getConstructors</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>2）获取所有的构造方法（包括私有、受保护、默认、公有）</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Constructor</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getDeclaredConstructors</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>3）获取一个指定参数类型的”公有的”构造方法</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Constructor</span> <span class="token function">getConstructor</span><span class="token punctuation">(</span><span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> parameterTypes<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>4）获取一个指定参数类型的”构造方法”，可以是私有的，或受保护、默认、公有</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Constructor</span> <span class="token function">getDeclaredConstructor</span><span class="token punctuation">(</span><span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> parameterTypes<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p><strong>使用开源库 Objenesis</strong></p>
<p>Objenesis 是一个开源库，和上述第二种方法一样，可以调用任意的构造函数，不过封装的比较简洁：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token punctuation">&#123;</span>   
    <span class="token comment">// 不存在无参构造函数 </span>
    <span class="token keyword">private</span> <span class="token keyword">int</span> i<span class="token punctuation">;</span> 
    <span class="token keyword">public</span> <span class="token class-name">Test</span><span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>    
        <span class="token keyword">this</span><span class="token punctuation">.</span>i <span class="token operator">=</span> i<span class="token punctuation">;</span>  
    <span class="token punctuation">&#125;</span>   
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">show</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>   
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"test..."</span> <span class="token operator">+</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>  
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span>   
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>   
    <span class="token class-name">Objenesis</span> objenesis <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ObjenesisStd</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>   
    <span class="token class-name">Test</span> test <span class="token operator">=</span> objenesis<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token class-name">Test</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  
    test<span class="token punctuation">.</span><span class="token function">show</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="反射获取成员变量"><a href="#反射获取成员变量" class="headerlink" title="反射获取成员变量"></a>反射获取成员变量</h2><p>和获取构造函数差不多，获取成员变量也分批量获取和单个获取。返回值通过 <code>Field</code> 类型来接收。</p>
<p><strong>批量获取</strong></p>
<p>1）获取所有公有的字段</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Field</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getFields</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>2）获取所有的字段（包括私有、受保护、默认的）</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Field</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getDeclaredFields</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p><strong>单个获取</strong></p>
<p>1）获取一个指定名称的公有的字段</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Field</span> <span class="token function">getField</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>2）获取一个指定名称的字段，可以是私有、受保护、默认的</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Field</span> <span class="token function">getDeclaredField</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>获取到成员变量之后，如何修改它们的值呢？</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">set</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">,</span> <span class="token class-name">Object</span> value<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalAccessException</span> <span class="token punctuation">&#123;</span> 
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>override<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>   
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">quickCheckMemberAccess</span><span class="token punctuation">(</span>clazz<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>     
            <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> caller <span class="token operator">=</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>    
            <span class="token function">checkAccess</span><span class="token punctuation">(</span>caller<span class="token punctuation">,</span> clazz<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">;</span>      
        <span class="token punctuation">&#125;</span>   
    <span class="token punctuation">&#125;</span> 
    <span class="token function">getFieldAccessor</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>通过这个 set 方法，就可以修改变量的值了，<code>set</code> 方法包含两个参数：</p>
<ul>
<li>obj：哪个对象要修改这个成员变量</li>
<li>value：要修改成哪个值</li>
</ul>
<h2 id="反射获取成员方法"><a href="#反射获取成员方法" class="headerlink" title="反射获取成员方法"></a>反射获取成员方法</h2><p>同样的，获取成员方法也分批量获取和单个获取。返回值通过 <code>Method</code> 类型来接收。</p>
<p><strong>批量获取</strong></p>
<p>1）获取所有”公有方法”（包含父类的方法，当然也包含 <code>Object</code> 类）</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Method</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getMethods</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>2）获取所有的成员方法，包括私有的（不包括继承的）</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Method</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">getDeclaredMethods</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p><strong>单个获取</strong></p>
<p>获取一个指定方法名和参数类型的成员方法：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Method</span> <span class="token function">getMethod</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">,</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> parameterTypes<span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>获取到方法之后该怎么调用它们呢？</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">invoke</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IllegalAccessException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">,</span> <span class="token class-name">InvocationTargetException</span> <span class="token punctuation">&#123;</span>  
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>override<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>       
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">quickCheckMemberAccess</span><span class="token punctuation">(</span>clazz<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>    
            <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> caller <span class="token operator">=</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>   
            <span class="token function">checkAccess</span><span class="token punctuation">(</span>caller<span class="token punctuation">,</span> clazz<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">;</span>      
        <span class="token punctuation">&#125;</span>   
    <span class="token punctuation">&#125;</span> 
    <span class="token class-name">MethodAccessor</span> ma <span class="token operator">=</span> methodAccessor<span class="token punctuation">;</span>       
    <span class="token comment">// read volatile  </span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>ma <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>     
        ma <span class="token operator">=</span> <span class="token function">acquireMethodAccessor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>   
    <span class="token punctuation">&#125;</span> 
    <span class="token keyword">return</span> ma<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> args<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p><code>invoke</code> 方法中包含两个参数：</p>
<ul>
<li>obj：哪个对象要来调用这个方法</li>
<li>args：调用方法时所传递的实参</li>
</ul>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>经过了上面发射的学习，我们应该对反射有了一定的了解，下面说说反射的优缺点：</p>
<p><strong>优点</strong>：比较灵活，能够在运行时动态获取类的实例。</p>
<p><strong>缺点</strong>：</p>
<p>1）性能瓶颈：反射相当于一系列解释操作，通知 JVM 要做的事情，性能比直接的 Java 代码要慢很多。</p>
<p>2）安全问题：反射机制破坏了封装性，因为通过反射可以获取并调用类的私有方法和字段。</p>
<p><strong>反射的经典应用场景</strong></p>
<ul>
<li>动态代理机制</li>
<li>使用 JDBC 连接数据库</li>
<li>Spring / Hibernate 框架（实际上是因为使用了动态代理，所以才和反射机制有关）</li>
</ul>
<blockquote>
<p>巨人的肩膀：</p>
<p><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzI0NDc3ODE5OQ==&amp;mid=2247485598&amp;idx=1&amp;sn=be7b37b1f75545ea9008c4d1f7a23524&amp;chksm=e959dd26de2e54301086a0f4226b735c74c9577563a2f92d4e71caadb98b95a382849912cbb5&amp;scene=178&amp;cur_album_id=1683346627601743872#rd">https://mp.weixin.qq.com/s?__biz=MzI0NDc3ODE5OQ==&amp;mid=2247485598&amp;idx=1&amp;sn=be7b37b1f75545ea9008c4d1f7a23524&amp;chksm=e959dd26de2e54301086a0f4226b735c74c9577563a2f92d4e71caadb98b95a382849912cbb5&amp;scene=178&amp;cur_album_id=1683346627601743872#rd</a></p>
</blockquote>

                
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